Gideon Jura's first ability doesn't lock in what it applies to. That's because the effect states a true thing about a set of creatures, but doesn't actually change the characteristics of those creatures. As a result, whatever creatures the targeted opponent controls during the declare attackers step of his or her next turn must attack Gideon Jura if able. This includes creatures that come under that player's control after the ability has resolved and creatures that have lost all abilities.

6/15/2010

Gideon Jura's first ability causes creatures to attack him if able. If, during the affected player's declare attackers step, a creature he or she controls is tapped, is affected by a spell or ability that says it can't attack, or is affected by "summoning sickness," then that creature doesn't attack. If there's a cost associated with having a creature attack, the player isn't forced to pay that cost, so the creature doesn't have to attack in that case either.

6/15/2010

If a creature controlled by the affected player can't attack Gideon Jura (because he's no longer on the battlefield, for example), that player may have it attack you, another one of your planeswalkers, or nothing at all.

6/15/2010

Gideon Jura's first ability applies during each combat phase of the affected player's next turn (as opposed to applying during the affected player's next combat phase). The distinction is relevant if there are no combat phases during that turn (due to Fatespinner's effect, for example) or there are multiples (due to World at War, for example).

6/15/2010

If Gideon Jura becomes a creature due to his third ability, that doesn't count as having a creature enter the battlefield. Gideon Jura was already on the battlefield; he only changed his types. Abilities that trigger whenever a creature enters the battlefield won't trigger.

6/15/2010

If Gideon Jura becomes a creature, he may be affected by "summoning sickness." You can't attack with him or use any of his {T} abilities (if he gains any) unless he began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when Gideon Jura came under your control, not when he became a creature.

6/15/2010

Gideon Jura's third ability causes him to become a creature with the creature types Human Soldier. He remains a planeswalker with the planeswalker type Gideon. (He also retains any other card types or subtypes he may have had.) Each subtype is correlated to the proper card type: Gideon is just a planeswalker type (not a creature type), and Human and Soldier are just creature types (not planeswalker types).

6/15/2010

If you activate Gideon Jura's third ability and then unpreventable damage is dealt to him (due to Unstable Footing, for example), that damage has all applicable results: specifically, the damage is marked on Gideon Jura (since he's a creature) and that damage causes that many loyalty counters to be removed from him (since he's a planeswalker). If the total amount of damage marked on Gideon Jura is lethal damage, he's destroyed as a state-based action. If Gideon Jura has no loyalty counters on him, he's put into his owner's graveyard as a state-based action.

6/15/2010

Say you activate Gideon Jura's third ability, then an opponent gains control of him before combat. You may have any of your creatures attack Gideon Jura (since he's still a planeswalker). Then Gideon Jura may block (since he's a creature). He may block any eligible attacking creature, including one that's attacking him! During combat, he behaves as an attacked planeswalker and/or a blocking creature, as appropriate. For example, he deals combat damage to any creatures he's blocking, but he doesn't deal combat damage to any unblocked creatures that are attacking him.

1/22/2011

The first ability only affects the declaration of attackers. If a creature is put onto the battlefield attacking (thanks to Hero of Bladehold, Preeminent Captain, or the Ninjutsu ability, for example), that creature's controller may choose the defending player or planeswalker that it will be attacking in the normal way.

7/1/2013

Planeswalkers are permanents. You can cast one at the time you could cast a sorcery. When your planeswalker spell resolves, it enters the battlefield under your control.

7/1/2013

Planeswalkers are not creatures. Spells and abilities that affect creatures won’t affect them.

7/1/2013

Planeswalkers have loyalty. A planeswalker enters the battlefield with a number of loyalty counters on it equal to the number printed in its lower right corner. Activating one of its abilities may cause it to gain or lose loyalty counters. Damage dealt to a planeswalker causes that many loyalty counters to be removed from it. If it has no loyalty counters on it, it’s put into its owner’s graveyard as a state-based action.

7/1/2013

Planeswalkers each have a number of activated abilities called “loyalty abilities.” You can activate a loyalty ability of a planeswalker you control only at the time you could cast a sorcery and only if you haven’t activated one of that planeswalker’s loyalty abilities yet that turn.

7/1/2013

The cost to activate a planeswalker’s loyalty ability is represented by a symbol with a number inside. Up-arrows contain positive numbers, such as “+1”; this means “Put one loyalty counter on this planeswalker.” Down-arrows contain negative numbers, such as “-7”; this means “Remove seven loyalty counters from this planeswalker.” A symbol with a “0” means “Put zero loyalty counters on this planeswalker.”

7/1/2013

You can’t activate a planeswalker’s ability with a negative loyalty cost unless the planeswalker has at least that many loyalty counters on it.

7/1/2013

Planeswalkers can’t attack (unless an effect turns the planeswalker into a creature). However, they can be attacked. Each of your attacking creatures can attack your opponent or a planeswalker that player controls. You say which as you declare attackers.

7/1/2013

If your planeswalkers are being attacked, you can block the attackers as normal.

7/1/2013

If a creature that’s attacking a planeswalker isn’t blocked, it’ll deal its combat damage to that planeswalker. Damage dealt to a planeswalker causes that many loyalty counters to be removed from it.

7/1/2013

If a source you control would deal noncombat damage to an opponent, you may have that source deal that damage to a planeswalker that opponent controls instead. For example, although you can’t target a planeswalker with Shock, you can target your opponent with Shock, and then as Shock resolves, choose to have Shock deal its 2 damage to one of your opponent’s planeswalkers. (You can’t split up that damage between different players and/or planeswalkers.) If you have Shock deal its damage to a planeswalker, two loyalty counters are removed from it.

7/1/2013

If a player controls two or more planeswalkers that share a planeswalker type, that player chooses one of them and the rest are put into their owners’ graveyards as a state-based action.